Mr Cameron yesterday flew into Camp Bastion, the main British base in Afghanistan, and proclaimed a “new phase” in Britain's decade-long military mission.

Despite his desire to announce a major reduction in British troop numbers, Mr Cameron accepted that UK numbers will see “no radical change” until next autumn.

That will give British commanders two “fighting seasons” to make military progress against the Taliban before the first UK combat troops are withdrawn.

The Prime Minister will on Wednesday tell the House of Commons that “relatively small numbers” of British troops will leave Afghanistan later this year.

That withdrawal is expected to involve fewer than 500 soldiers, and leave the overall British force level broadly unchanged at 9,500. Significantly, no combat units will be affected. A larger withdrawal will then follow next autumn.

British commanders have resisted Mr Cameron’s drive for a major withdrawal, warning that too hasty a pullout could jeopardise hard-fought progress in Helmand province.

Asked if any combat troops will leave Afghanistan before next autumn, Mr Cameron indicated that they would not.

He said: “You have the enduring number of 9,500. You’re not going to see a radical change for the fighting season of next year. That’s not what this is about.”

British commanders were privately pleased with Mr Cameron’s remarks, believing they have curbed his push for a larger withdrawal. “We’ve had a bit of a victory on numbers – we’ve got to the right place,” said one senior officer.

By pushing back much of his withdrawal until the autumn of 2012, Mr Cameron is proceeding less quickly than US President Barack Obama, who wants to withdraw tens of thousands of American troops next summer.

Despite his apparent concessions on his immediate withdrawal plans Mr Cameron remained adamant that his 2015 deadline for ending combat operations will not slip.

“The end of 2014 is a deadline - be in no doubt,” he said. “This is a matter of judgment. It is my judgement that it is right.”

British forces first arrived in Afghanistan in 2001. Since then, at least 374 British service personnel have died and the Treasury has spent more than £14 billion.

After so much time and effort, a clear end to the mission is required, Mr Cameron said. He said: “The British people and the British military deserve to have some certainty.”

Brigadier Ed Welch, the deputy commander of Nato forces in Helmand, said that Mr Cameron’s plans will mean “thinning out” his forces in parts of the provinces, but said that the Prime Minister’s plan is “doable”.

However, he also said that even after Mr Cameron’s deadline, British forces will remain closely involved in Afghan security operations against the Taliban.

“We will be close by to provide the necessary support,” Brg Welsh said. British forces will have to help the Afghans with the “combining of fire” and the control and co-ordination of aircraft and helicopters.